Several processes used in the assembly of printed circuit assemblies (PCAs) require a means to support a PCA or module. This is particularly important during operations which apply force or pressure to an opposing side of the module. A supporting surface of the module may have components attached, forming a three dimensional surface.
One known method used for supporting the module is to machine custom contoured support plates designed to accommodate a three dimensional surface. There are several reasons why this solution is not desired. First, the time to manufacture the support plate reduces the Time-To-Market for the product. Secondly, the tooling plates are expensive. Thirdly, additional set up is required for each production run, significantly adding to the cost of the product.
A second known method used for supporting a module is to place fixed height, support members proximate to locations on the module that are substantially on the same plane. Utilizing this method for a PCA with bottom side parts, elongated support members would be placed in areas where no parts were present to support the planer PCB while not contacting or damaging the components. This process is difficult to repeat, and costly to automate (U.S. Pat. No. 5,794,329, Rossmeisl, et al). The PCA must be designed such that there is sufficient clearance for the contact area of the supporting pins. For an automated placement system, the system must be programmed, which is time consuming and may include errors.
A third known method described by Beale, U.S. Pat. No. 5,157,438, teaches the use of individual armatures and electromagnets magnets to selectively clamp the elongated support members in a raised position or allow them to fall to a lowered position. The elongated support members are either fully extended or fully retracted. This method requires complex designs and systems to support it. If the elongated support members do not align with areas on the said module that are planer with the module, the elongated support members will fall to the lowered position, thus not adequately supporting the said module.
A fourth known method described by Barozzi, U.S. Pat. No. 4,936,560, discloses an assembly used to support a module utilizing support pins which are restrained via application of a frictional force to the pins. Barozzi describes an assembly that utilizes a pair of plates to mount a number of pins. The pins are restrained in a home position using a frictional force. A workpiece on a machining line is then impressed upon the pins to form a three dimensional support fixture. After the workpiece is machined, an additional step is required to reset the pins to the home position utilizing a separate mechanism. There are a number of disadvantages to this method. First, the frictional force applied to the sides of the pins is difficult to control and would not be suitable for the manufacture of electronic modules. Over time, dirt, solder paste and other materials used in the assembly of modules may fall down onto the assembly changing the coefficient of friction between a lamallar spring and the pins. This additional force may damage the module. Once the pin begins to move, the frictional force is reduced, as it changes from static to dynamic friction, again making it more difficult to control. The additional force required to initiate pin movement may damage the module suggesting a serious disadvantage. Secondly, the pins would be subject to wear causing a change in the coefficient of friction between the lamallar spring and the pins. This type of wear would cause the pins to fall and not properly support the workpiece. Thirdly, the assembly has no automatic reset mechanism. This is a major deficiency because of the time required to reset the assembly utilizing a separate mechanism. Fourthly, the assembly utilizes two locking plates adding additional cost to the assembly. Lastly, the pins are fluted on the ends, disallowing removal when required by the profile of the module or the proximity to fragile components. Removal of pins is also necessary in electronics manufacturing so that the pins and the assembly may be cleaned and serviced.
A fifth known method described by Dougherty, U.S. Pat. No. 5,152,707, discloses an assembly used to support a color cathode ray picture tube, having a substantially flat face panel. Dougherty teaches a supporting anvil which uses a plurality of elongated support members which move axially through a plurality of split sleeves. The split sleeves are encased a plurality of flexible tubes. The plurality of split sleeves encased in flexible tubes are located within a chamber. The chamber pressurizes, where the pressure causes the flexible tubing to collapse the split sleeves, where the split sleeves apply pressure to the elongated support members, thus securing them in place. Dougherty utilizes pressurized fluid to urge the elongated support members into near pressureless (10 lbs/in.sup.2) contact with the panel through the pressurization of a second chamber. The process flow includes locating the module, pressurizing the second chamber to raise each individual elongated support member to contact the panel. Subsequent to contact, the first chamber is pressurized to secure the elongated support members, thus supporting the module. The module is then processed, and removed.
Dougherty, while contributing to the art, clearly has disadvantages when applied to electronics module manufacturing. State of the art solder screen printers, component placement equipment, and dispensers have a mechanical table which is actuated when the module is in position. The table in each of these cases, raises to place pins or other supporting devices, described above, proximate the module. Dougherty discloses individual pins raising to a flat panel. If used in conjunction with a raising table, additional proximity switches, sensors, logic and actuators would be required to ensure no damage is done to the module. Additionally, cycle time is critical in high speed assembly lines such as those used to manufacture electronic modules. The additional process time required to raise the elongated support members to the module after locating the module and raising the table, would require more time than economically allowed. Secondly, the nearly pressureless force of 10 lbs/in.sup.2 is far greater than generally accepted limit of 250 grams (0.55 lbs) of applied force to components. The use of pressure dictates the requirement to seal the first chamber, including a seal around the elongated support members. The seal will introduce friction. The air pressure must overcome the gravitational forces of the elongated support members and the friction of the seal. The friction may vary between different elongated support members exerting different forces over the surface of the module. The seal will wear over time lowering the friction and changing the force applied to the module. Dougherty also includes a small continuous leakage to avoid entry of abrasives or slurries to minimize damage to these seals. The use of a pressure chamber increases the complexity of the apparatus should the system utilize vacuum to secure the module above the self conforming apparatus. Vacuum is often used to secure Printed Circuit Boards (PCB) during several processes, particularly as a means to remove any warpage. Thirdly, if one of the elongated support members were removed as is often required to preclude damage to sensitive components, the seal would be broken to the chamber making the pressurized lifting system inoperable. This limits the flexibility of the system for certain assemblies with sensitive components and is clearly a disadvantage. The use of pressure chambers, additional components, sensors, proximity switches, computer logic, and actuators are clearly not desired. Additionally, the use of a pressurized lifting chamber is not conducive to applications which require very narrow self conforming systems, such as the use in conveyorized, automated assembly equipment where there is limited distance between the conveyor and the base of the machine. Fourthly, the cycling of the split sleeves may cause them to permanently deform over time causing additional friction to the elongated support members, where the split sleeves would have to be replaced. The process to replace the split sleeves is time consuming and difficult, as they require breaking and recreating good air pressure seals.
Thus, what is desired is a method of supporting planer and non-planer modules that can create a profile to support a desirable face of any module without applying excessive forces to the module. The system should be designed to continuously operate with minimal cycle time, while being repeatable, serviceable, and should have minimal complexity.